Many motor vehicle electrical systems are now being designed with a dual voltage schemes requiring two batteries having nominal voltages of 14V and 42V (12V and 36V rated batteries respectively) as shown in FIG. 1. The 12V battery 40 typically has a high amp-hour rating and is used to provide energy to 14V loads 50 such as lighting circuits and other circuits which are difficult to implement at higher voltages. The 36V battery 80 typically has a high cranking current capability and is coupled to a 42V generator and higher voltage loads 70, which may include the engine starter motor.
In the event that one or other of these batteries becomes depleted of charge, there is a need to transfer power between them in a bidirectional manner. In order to do this, it is known to provide a conventional bidirectional DC-DC converter 60, coupled between the 12V battery 50 and the 36V battery 80. The bidirectional DC-DC converter 60 acts as a step-down converter (right to left in FIG. 1) or a step-up converter (left to right in FIG. 1) through switching charge through an inductor in a well known manner.
An external `start aid` post 10 is also provided, to enable an external means of charging the batteries. A switch 30 switches between the start aid post 10 and the 12V battery 40, and a fuse and diode arrangement 20 is coupled between the switch 30 and the start aid post 10. When a positive DC voltage is applied to the start aid post 10, the switch 30 isolates the 12V battery 40 and the DC voltage is coupled through the fuse and diode arrangement 20 to charge the 36V battery 80 via the bidirectional DC-DC converter 60. When the DC voltage is removed from the start aid post 10, the switch 30 isolates the start aid post 10 and re-couples the 12V battery 40 to the bidirectional DC-DC converter 60, whereupon (if necessary) the 12V battery 40 is charged by the 36V battery 80 via the bidirectional DC-DC converter 60.
FIG. 2 shows the internal architecture of the bi-directional DC-DC converter 60, which has a first path 100 coupled to the 36V battery 80 (not shown), a second path 170 coupled to the 12V battery 40 (not shown), first and second switches 130 and 150 respectively and an inductor 140. The first and second switches 130 and 150 respectively are coupled in series between the first path 100 and earth. The inductor 140 is coupled between the second path 170 and a node between the first and second switches 130 and 150 respectively. The switches are switched by control logic in one of two ways: to transfer energy from the first path 100 to the second path 170 (step-down); and to transfer energy from the second path 170 to the first path 100 (step-up). Both of these are achieved by switching charge through the inductor 140.
A problem with this arrangement is that for it to function correctly as a step-up converter, the first path 100 (and hence the 36 battery 80) must be at a higher potential than the second path 170, otherwise the intrinsic body diode 135 of the first switch 130 will conduct. Therefore if the 36V battery 80 is faulty, greatly discharged or replaced by a new battery, and therefore has a voltage less than that of the 12V battery 40 (or the start aid post 10, if appropriate), then the current flow will be uncontrolled, with potentially catastrophic results. It is possible to prevent this current flow by adding another switch in inverse series with the first switch 130, but this would still not enable charging in this state. This problem is compounded by the emergence of vehicles with an exclusively 42V electrical system, because such vehicles cannot be used to provide a jump-start via the start aid post 10.
A further problem is that by adding an additional switch the DC-DC converter 60, the circuit of FIG. 1 would require 7 MOSFETs (metal-oxide semiconductor field-effect transistors), as the changeover switch in the start aid post 10 requires 2 sets of inverse series MOSFETs, in addition to the three required in the DC-DC converter 60.
There is therefore a need for a more flexible arrangement which enables a two-battery vehicle to re-charge either battery from the other, and which also provides improved flexibility for to charge and be charged via a start aid post.
This invention seeks to provide a DC-DC converter and energy management system which mitigate the above mentioned disadvantages.